The present invention relates to a multilayer laminated structure suitable for use in particular in wrapping electric cables in order to provide electrical insulation.
A particular application lies in aviation cabling where it is essential to provide very good electrical insulation so as to prevent any propagation of an electric arc.
In that type of application, use has been made for many years of multilayer laminated structures implementing a layer of polyimide such as xe2x80x9cKaptonxe2x80x9d from the supplier E.I. du Pont de Nemours and Co. That material presents good high temperature performance and good electrical insulation performance.
An example of such a multilayer structure is described in that supplier""s U.S. Pat. No. 3,616,177. That structure comprises a layer of polyimide coated on at least one of its faces in a layer of fluorinated ethylene propylene (FEP) and a layer of polytetrafluoroethylene (PTFE) bonded to one of said FEP layers. The PTFE layer is applied by being coated in the form of an aqueous dispersion.
The presence of FEP serves to ensure that the polyimide on which it is applied does not absorb external moisture which could lead to it becoming degraded by hydrolysis.
A drawback of such a multilayer structure lies in the fact that the PTFE layer applied by coating an aqueous dispersion presents thickness that is small, providing little strength and little resistance to abrasion.
Unfortunately, when wrapping using modern taping machines that can rotate at speeds in the range 2000 revolutions per minute (rpm) to 3000 rpm, it is necessary to have a structure that can withstand the rate at which taping is performed, and thus a structure in which all of the layers, and in particular the PTFE layer, present sufficient strength to be able to withstand that rate of taping without faults being generated.
The present invention provides a multilayer laminated structure comprising both a polyimide layer coated on at least one of its faces in FEP, and at least one layer of PTFE bonded to said FEP-coated polyimide layer, wherein said layers are constituted by tapes that are bonded together while hot with pressure being applied, the or each PTFE tape being made of green PTFE obtained by lubricated extrusion and being united with the FEP-coated polyimide tape at a temperature that is lower than the sintering temperature of PTFE.
Thus, in the final product, the PTFE is green, and the tapes are united at a temperature which lies in practice in the range 280xc2x0 C. to 300xc2x0 C., and which is higher than the melting point of FEP so as to cause the FEP to melt and bond the tape(s) of PTFE to the FEP-coated polyimide tape.
In the invention, the temperature during heating while pressure is being applied is lower than 340xc2x0 C. which is the sintering (gelling) temperature of PTFE.
Advantageously in the invention, only one tape of PTFE is provided and the width of the PTFE tape is greater than the width of the FEP-coated polyimide tape.
The PTFE tape preferably projects sideways from only one side of the FEP-coated polyimide tape. This makes it possible during taping to provide improved isolation of the FEP-coated polyimide layer, with the projecting margin of the PTFE tape constituting a continuous barrier that protects the polyimide from possible spoiling in an aggressive environment.
The PTFE used in the invention is obtained by lubricated extrusion so a to present a fibrillated structure that is oriented in the longitudinal direction of the resulting tape.
A lubricated extrusion method consists in mixing PTFE powder, together with a filler and/or pigments, if any, and a lubricant so as to provide a compact preform which is subjected to pressure extrusion in which a piston urges the optionally-filled PTFE through a die so as to form an extrusion of constant section, such as a flat tape or a cylindrical rod, the lubricant subsequently being eliminated, generally by heating.
The resulting tape is generally calendared and, where appropriate, it may be subjected to a de-densifying operation by stretching that imparts inter-fibril porosity to the final product.
The tape of the invention may also be of the type that is suitable for being marked by laser, e.g. having the structures described in French patents 92/06427 and 95/03195 in the name of the Applicant company.